3/4 vs 4/4, recommendations?

I've been playing electric bass for a few years, and am looking into purchasing a double bass, mostly for rockabilly playing with alot of slapping. I've come across 3/4 and 4/4 scales, and i was wondering what would i base a decision on? I am about 6'5", that might be a factor?

You probably want to go with a "standard" or 3/4 size. (As noted above, concentrate on scale length rather than the 5/8, 3/4, 7/8 designation.) The biggest advantage of a bigger-bodied bass is that it can project more volume acoustically. Since you are probably going to be playing amplified, you are more interested in resistence to feedback than maximum acoustic volume.

The scale difference coming from EB is already going to be a pretty big adjustment, don't make it harder on yourself. I've come across very few people who play or have played a 4/4 bass.

And, I'd be remis if I didn't say "get a teacher". even if it's just for a few lessons to show you the traps of making the shift. It will save you a year of fooling around trying to figure out why you're never in tune and how come your hands keep cramping up.